When a combine harvester with a straw walker that operates at a fixed oscillation frequency travels uphill, as a consequence of this, the transport velocity of the crop on the straw walker will be increased, leading to the risk of increased grain loss as time is shortened during which the crop material is handled by the straw walker. When such a combine harvester is traveling downhill, the transport velocity of the crop on the straw walker will decrease and there will be a decrease in throughput and also a risk for the formation of plugs as the crop material takes more time to travel along the straw walker as the crop material layer on the straw walker will become too thick, which also is not optimal for separation of the grain and could thus also lead to increased grain loss.
It is proposed in M. Gubsch, “Der Einfluss der Längsneigung auf das Abscheidungs-und Förderverhalten des Strohschüttlers”, Archiv der Landtechnik Band 8 (1969), part 2/3, pages 127-139, to change the oscillation frequency of the straw walker in function of the longitudinal inclination of the combine harvester in order to counteract the effect of the longitudinal inclination. However no embodiment of such a straw walker drive system and associated control system was proposed.
EP1584225 proposed a straw walker drive system and associated control system that enabled control of the oscillation frequency as well as the stroke of the oscillation of the straw walker drive system. However such a combine harvester requires an intermediate frame which complicates the construction of the straw walker drive system and makes it almost impossible to retrofit an existing combine harvester with such a system. Furthermore the control system makes use of a correlation between the throughput of the crop material and an optimal transport velocity for this particular throughput of crop material. For assessing the throughput of the crop material there are required throughput sensors that assess the amount of crop material that is handled during a predetermined time period by the combine harvester. It has been found that such a control system is not optimal as it lacks the necessary precision because the signal provided by the throughput sensors is very noisy. Furthermore some embodiments of these sensors require the sensors to be arranged in close proximity to the crop flow, which subjects them to hostile operating conditions comprising long term contact with dirt, moisture, dust, etc. even further reducing long term reliability of the measurements of these throughput sensors. It has also been found that responsiveness of the control system in some circumstances can be too low, mainly because of the inertia of the straw walker drive system and the fact that the throughput sensors measure the throughput of the crop material when it is already being processed by the combine harvester. In this way the straw walker control system might not be able to handle sudden changes in throughput efficiently, such as for example occurring when a combine travels past the lowest point of a valley where its longitudinal inclination rapidly changes from a downward slope to an upward slope. Throughput sensors arranged in the feeder will not even notice such a change if the combine keeps on travelling at the same speed as the amount of incoming crop material at the header and feeder will not change noticeably. However this change in longitudinal inclination will instantly increase the transport speed of the crop material on the straw walker which eventually could cause increased grain loss or a plug. Although these changes can be detected with embodiments of throughput sensors which measure the thickness of the crop material layer on the straw walker or the grain loss sensors arranged at the end of the straw walker, such an assessment will only be made when the effect is already clearly available at the level of the straw walker, and in the case of the grain loss sensor, at the end of the straw walker. So when additionally taking into account the delay caused by the inertia of the oscillating straw walker, such a control system will not be able to deal timely with such sudden changes and additionally such delays will cause the control system to divert considerably from the optimal set-point, and overshoot it considerably during the time period following such changes.
A further straw walker drive system with controllable oscillation frequency is shown in DE102005050751. This straw walker drive system does not require an intermediate frame and makes use of a belt variator in the straw walker drive system, which simplifies the construction considerably. However it still remains cumbersome to retrofit such a system to an existing combine harvester as it requires at least replacement of the pulley that is mounted to the crankshaft of the straw walker drive system of the combine harvester with a belt variator. This operation is complex and requires a bulky design of the belt variator. Additionally the straw walker drive control system requires the use of sensors that assess the transport speed of the crop on the straw walker during operation. Such sensors are generally not available in existing combine harvesters and thus lead to major modifications to the combine harvester control system in order to connect to such sensors. Furthermore such sensors operate in extremely harsh conditions which affects their reliability and precision. On top of that these sensors assessing the transport velocity are arranged at the height of the straw walker and also here the effect will only be detected when it is already clearly available on the straw walker, which when taking into account the delay caused by the inertia of the oscillating straw walker will lead to diversion of the optimal set-point and overshoot, especially in the case of a sudden change in the longitudinal inclination of the combine as explained above. Additionally the control system aims to achieve a constant transport velocity for the crop material on the straw walker. It has been found that maintaining such a constant transport velocity is not able to guarantee an efficient operation of the straw walker, especially if changes in the longitudinal inclination of the combine harvester occur. This is because at different longitudinal inclinations of the combine harvester the mean angle at which the crop material is excited by the straw walker elements changes, and thus when the time period during which the crop material is handled by the straw walker is kept constant, the effect of the excitation by the straw walker elements will be different when the longitudinal inclination of the combine harvester changes, still further leading to diversion of the optimal set-point of the control system.
Thus there still remains a desire for a straw walker drive and associated control system for a combine harvester that overcomes the above mentioned drawbacks, is able to ascertain an increase in the long-term efficiency of the operation of the straw walker by means of a control system with a higher precision and increased responsiveness, is of a simple construction, can be easily retrofitted to existing combine harvester designs and does not require the use of throughput sensors or transport velocity sensors at the height of the straw walker.